Karen Reichstein thought she had a co-worker's buttoned-down personality pegged. Then she used her computer to peek into his iTunes digital music library and found some surprises.

"He had a huge collection of the Jam and the Kinks," said Reichstein, an associate editor for a Berkeley book publisher. "Here was a person next to me who even had the whole Carter family collection" of country music.

The old adage used to be "you are what you eat." But with the advent of digital music and the popularity of gadgets like the iPod, now it's "you are what's on your playlist."

Playlists are groups of songs a person can tailor to his or her own tastes or moods for playback on an MP3 player or computer. Those tunes are picked from the larger library of music that a person can store on a portable player or computer.

Last week, musicologists and media pundits around the world had a great time trying to divine what makes President Bush tick by analyzing the songs loaded on his iPod.

The presidential playlist included John Fogerty's "Centerfield" and "Swinging From the Chains of Love" by Blackie & the Rodeo Kings.

But playlist watching has also become a parlor game played by college students and office workers hoping for insight into the lives of people around them. They use a feature in Apple Computer Inc.'s iTunes digital music management program that allows a limited number of people to surf and hear songs in someone else's library.

Playlist peeking isn't limited to your neighbors. A number of famous iTunes consumers have published their lists of favorite songs on the iTunes Music Store site, including San Mateo's Tom Brady, quarterback for the New England Patriots, and Broadway composer Andrew Lloyd Webber. (Brady's list ranged from Aerosmith's "Dream On" to "If I Can't" by 50 Cent. Meanwhile, Lloyd Webber's tastes ran from Elvis to Eminem.)

Experts say these playlists and digital music libraries may even become a new way for people to size up potential mates or political candidates.

"We do find that people are able to make fairly accurate assessments solely on the basis of a person's top 10 songs," said Jason Rentfrow, a psychology consultant who co-authored a 2003 University of Texas study of more than 3,500 people that showed musical taste can provide a road map to a person's personality.

Throughout history, music has always been a shared human experience. But what's different now is that technology is allowing people to aggregate their entire musical collections in one place, whether it's on a portable digital music player or in a music management program -- like iTunes -- on a computer.

So while it used to take hours to browse through a person's CD collection one album at a time, technology now makes that possible with a few clicks of a mouse or the twirl of a scroll wheel.

"It's one of the most intimate things you can create," said Reichstein. "It's like a small diary made up of your songs. You feel like you know them more than you actually do."

Reichstein was one of 34 contributors to "The iPod Playlist Book," published by her company, Peachpit Press, in December.

In a report released earlier this month, researchers from the Palo Alto Research Center, known as PARC, and the Georgia Institute of Technology studied 13 workers at one small company and found they were forming judgments about co-workers based on the songs they found in each others' iTunes music libraries.

The iTunes program, which works with Windows and Macintosh computers, has a built-in feature designed for users inside one household to stream music from one computer to another as long as they are hooked up to a closed home network.

Researchers Amy Voida, Beki Grinter and Keith Edwards began tracking stories about how college students were using the same iTunes sharing feature to form musical communities on campus networks and found a company in which employees were doing the same.

At the company, the employees became aware that their music was projecting an image of themselves to co-workers, Grinter said in an interview.

That caused some playlist anxiety. One worker said he was worried others would get the wrong impression because he downloaded songs by Justin Timberlake and Michael McDonald for his wife. Another worker reported sensing his library was not very cool, so he added artists and genres to make himself appear more balanced.

Some libraries had only the default iTunes name, so its owner remained anonymous, but that only fueled a new game in which the workers tried to match the type of songs listed with co-workers.

"People spent a lot of time trying to figure out who (owned) these anonymous collections," Grinter said.

Christopher Breen, editor in chief of Playlist, a San Francisco magazine covering the digital music scene, said he did some eaves-sharing at a recent music industry conference in Texas, using his laptop to scan a dozen iTunes libraries of other conventioneers.

He was surprised to find one acquaintance had a library filled with schmaltzy country, while another had Celtic bagpipe music.

Breen said he keeps his own library in pristine condition.

It's similar to when "your mother tells you always to wear clean underwear because you never know if you're going to end up in the emergency room," he said. "Now you have to worry if someone sees you have 'Me and You and a Dog Named Boo' in your music collection."

Rentfrow, who with Samuel Gosling authored the study on music and personality that was published in the Journal of Personality and Social Psychology, said the research showed people have an intuitive feel for matching musical tastes with someone's personality.

"People who were more extroverted tended to have songs in their music collection that had lots of singing," said Rentfrow, who is now an online dating services consultant but is set to become a professor at the University of Cambridge in the fall. People who leaned toward instrumental music, meanwhile, were more introverted.

"This suggests he's predominantly someone who is extroverted and a bit relaxed, but not necessarily open to new experiences or is unconventional by any means," Rentfrow said. "These are more conventional styles of music."

His preferences for rock and country "are related to his athletic ability ... but also goes negative toward political liberalism," he said.

Sharing playlists and musical libraries has even become a new method for men and women to flirt.

"She was sitting across the coffee shop, and I didn't know who she was," said Alexander Payne, 21, recalling a brief but memorable virtual encounter he had one evening in February in his favorite Arlington, Va., java house.

Payne had fired up iTunes when a library titled "Maria's music" appeared from another laptop using the same network. "She had excellent taste, music I hadn't seen in too many other people's libraries, stuff I had been looking for a long time," said Payne, an information security contractor.

Not knowing who she was, Payne changed the name of his own iTunes library to read, "Maria I sweat your music collection." Soon, "Maria's music" changed to, "trythenew_dalek." The two carried on a two-hour conversation by changing library titles.

Payne, however, said he never talked to Maria. He saw her at the coffeehouse one other night, but said she "looked like she didn't want to be disturbed."

"It's almost a better experiment unfinished," Payne said last week. "A modern love story, only without a happy ending."